1. Field of the Invention
The present invention relates to a receiver of a wireless communication terminal working in a wireless communication environment. More particularly, the present invention relates to a direct conversion receiver adopting direct conversion and a direct conversion reception method.
2. Description of the Related Art
In general, heterodyne-type receivers have been used in mobile phones, such as cellular phones or personal communication service (PCS) phones, and wireless phones that provide wireless communications.
FIG. 1 is a diagram illustrating a conventional super heterodyne method where a low frequency signal containing data, such as voice or image data, is converted into an intermediate frequency signal and then the intermediate frequency signal is transmitted on a radio-frequency (RF) carrier. A super heterodyne receiver adopting the super heterodyne method is required to include a frequency converter for converting an RF signal into an intermediate frequency signal, a frequency converter for converting the intermediate frequency signal into a baseband signal, and a band pass filter for processing signals in different frequency bands.
Extensive research has been performed on wireless communication terminals in an effort to solve such disadvantages of a heterodyne receiver. Resultantly, an alternative to the heterodyne receiver has been developed. The alternative is a receiver adopting a direction conversion method.
FIG. 2 is a diagram illustrating a conventional direct conversion method where a low frequency signal containing data, such as voice or image data, is directly transmitted on an RF carrier without being converted into an intermediate frequency signal. A direct conversion receiver adopting the direct conversion receiving method enables a local oscillator to operate at the same frequency as that of an RF signal input thereinto via an antenna and converts an RF signal into a baseband signal without a process of converting the RF signal into an intermediate frequency signal.
FIG. 3 is a diagram illustrating a leakage signal generated in a local oscillator according to the prior art. Referring to FIG. 3, during a decreasing of the frequency of an RF signal to a baseband level, there exists a moment when a local oscillator (LO) 330 has a same frequency as that of the RF signal. At this moment, frequency leakage 310 not associated with any kind of signal may occur in the local oscillator 330. In particular, when a leakage signal of the local oscillator 330, which has been amplified through a low noise amplifier (LNA) 320, is mixed with the original frequency of the local oscillator 330 in a mixer 340, a DC signal having an arbitrary value is output. Even though there exists a filter 350 between the low noise amplifier 320 and the mixer 340, there is a limit in reducing such frequency leakage in the local oscillator 330 because the local oscillator 330 has the same frequency as that of the RF signal.
FIG. 4 is a diagram illustrating interference leakage according to the prior art. When a strong signal having a frequency level different from that of a local oscillator (LO) 430 is input, frequency leakage occurs in the local oscillator 430, and thus a DC offset signal can be generated in a mixer 440. The DC offset signal damages a signal intended to be demodulated. Similar to FIG. 3, FIG. 4 further includes a low noise amplifier (LNA) 420 and a filter 450.
There are several techniques known in the art to solve the above problem with interference leakage. In pulsed-mode communications using a timeslot, such as GSM, interference leakage is prevented by discharging DC charge when there is no communication service provided. In wireless local area network (LAN) communication, an orthogonal frequency division multiplexing (OFDM) technique, where no signals are loaded in a DC frequency range, is adopted to solve the interference leakage problem. In code division multiple access (CDMA) communication, which uses even more complicated modulation techniques than pulsed-mode communication and wireless LAN communication, a complex self-calibration method is adopted to suppress interference leakage.
In general, a direct conversion receiver (DCR) has two channels, i.e., an I-channel and a Q-channel. The I-channel and the Q-channel each have a mixer, a baseband filter, and a baseband amplifier. Since the elements of the I-channel are not exactly the same as those of the Q-channel, however, they show different gain responses and phase responses in a baseband frequency as compared to counterparts thereof.
FIG. 5 is a block diagram of a conventional direct conversion receiver. Referring to FIG. 5, an incoming band pass signal is received at an RF input port and then passed through an RF fliter 510, a preselector filter 520 and a low noise amplifier (LNA) 530. The preselector filter 520 is simply a band pass filter designed to pass a desired signal and to reject spurious out-of-band signals.
After passing through the preselector filter 520 and the low noise amplifier (LNA) 530, the incoming signal is split and sent through an upper mixer 540 and a lower mixer 550. In the upper mixer 540, the corresponding signal is mixed with a sinusoid tuned to the same frequency as a carrier frequency. In the lower mixer 550, the corresponding signal is mixed with the same sinusoid as in the upper mixer 540, but with a phase difference of 90°. The sinusoids are generated by a local oscillator (LO) 555. The upper and lower mixers 540 and 550 produce in-phase and quadrature components, respectively, of the corresponding signal, which are centered at the based band and at twice the carrier frequency. High frequency components are eliminated by filters (e.g., IF filters) 560 and 570, and the in-phase and quadrature signals are finally amplified by amplifiers 580 and 590, respectively, and then become an I-channel signal and a Q-channel signal, respectively.
More particularly, one of the above-described conventional techniques, a super heterodyne receiver, includes many elements. Thus, the super heterodyne receiver is not appropriate for wireless communication terminals, such as mobile phones, because the size of wireless communication terminals has continued to decrease in recent years. In addition, the manufacturing cost of the super heterodyne receiver is high because of the large number of elements constituting the super heterodyne receiver. Moreover, mobile phones have been improved to provide multimedia services as well as voice calls, and the elements thereof or other related circuits have been required to have a smaller size and a more simplified structure. Therefore, there is a limit in adopting the super heterodyne receiver in such wireless communication terminals.
A conventional direct conversion receiver does not include any elements for processing intermediate frequency signals and may be appropriate for mobile phones required to meet the above-described demands. However, manufacturing costs of a communication system using intermediate frequencies is high because the communication system is required to include many elements, including filters, amplifiers, and peripheral circuits, in order to perform intermediate frequency conversion. Specifically, communication techniques need to be capable of directly shifting carrier frequency to baseband frequency or baseband frequency to carrier frequency. However, such direct conversion causes many problems, and thus there is a limit in improving the performance of a communication system adopting direct conversion. More specifically, the conventional direct conversion receiver has a receiver of a simple structure but causes various problems with oscillation, a selectivity level, and a DC offset.
In addition, differences in gains and phase responses between an I-channel and a Q-channel cause mismatches therebetween. The direct conversion receiver cannot completely eliminate an inverse spectrum of an incoming signal due to mismatches between the I-channel and the Q-channel, which causes a significant interference ratio loss. Due to interference ratio loss, a bit error rate increases.